The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing the migration of charged pigment particles in a solvent, preferably a colored dielectric solvent. This type of display was first proposed in 1969. An EPD typically comprises a pair of opposed, spaced-apart plate-like electrodes, with spacers predetermining a certain distance between them. At least one of the electrodes, typically on the viewing side, is transparent. For the passive type of EPDs, row and column electrodes on the top (the viewing side) and bottom plates, respectively, are needed to drive the displays. In contrast, an array of thin film transistors (TFTs) on the bottom plate and a common, non-patterned transparent conductor plate on the top viewing substrate may be used for the active type EPDs.
An electrophoretic dispersion composed of a dielectric solvent and charged pigment particles dispersed therein is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the charged pigment particles migrate by attraction to the plate of polarity opposite that of the pigment particles. Thus, the color showing at the transparent plate, determined by selectively charging the plates, may be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite plate, thereby reversing the color. Intermediate color density (or shades of gray) due to intermediate pigment density at the transparent plate may be obtained by controlling the plate charge through a range of voltages or pulsing time.
EPDs of different pixel or cell structures have been reported previously, for example, the partition-type EPD [M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., Vol. ED 26, No. 8, pp. 1148-1152 (1979)], the microencapsulated EPD (U.S. Pat. Nos. 5,961,804 and 5,930,026 and U.S. applications Ser. No. 60/443,893, filed Jan. 30, 2003 and Ser. No. 10/766,757, filed on Jan. 27, 2004) and the total internal reflection (TIR) type of EPD using microprisms or microgrooves as disclosed in M. A. Mossman, et al, SID 01 Digest pp. 1054 (2001); SID IDRC proceedings, pp. 311 (2001); and SID'02 Digest, pp. 522 (2002).
An improved EPD technology was disclosed in co-pending applications, U.S. Ser. No. 09/518,488, filed on Mar. 3, 2000 (WO01/67170), U.S. Ser. No. 09/606,654, filed on Jun. 28, 2000 (WO02/01281) and U.S. Ser. No. 09/784,972, filed on Feb. 15, 2001 (WO02/02/65215), the contents of all of which are incorporated herein by reference in their entirety. The improved EPD comprises isolated cells formed from microcups and filled with charged pigment particles dispersed in a dielectric solvent or solvent mixture. To confine and isolate the electrophoretic dispersion in the cells, the filled cells are top-sealed with a polymeric sealing layer, preferably formed from a composition comprising a material selected from a group consisting of thermoplastics, thermoplastic elastomers, thermosets and precursors thereof.
A liquid crystal display comprising display cells prepared by the microcup technology and filled with a liquid crystal composition optionally comprising a dichroic dye is disclosed in a copending U.S. application, Ser. No. 09/759,212, filed on Jan. 11, 2001, the content of which is incorporated herein by reference in its entirety.
For a direct drive display, a display cell layer (10) typically is sandwiched between a common electrode layer (11) and a backplane (12) as shown in FIG. 1a. 
The common electrode layer (11) is a single electrode layer which covers the entire display area. The backplane (12) comprises a substrate layer on which a desired graphic pattern (13) is printed or coated with a conductive material. The charged pigment particles in the display cell layer in the area of the desired graphic pattern may migrate to either the side of the common electrode layer or the side of the backplane, depending on the voltage difference between the common electrode layer and the conductive pattern. The color of the non-patterned area (i.e., the background color) is usually achieved by color matching.
The desired graphic pattern may be alphabet letters, numerical displays (such as those utilizing the well-known 7 or 14 segment electrodes), logos or any other graphic designs. Each of the graphic patterns on the substrate layer has a lead line connecting the pattern to associated components such as driver and/or control elements. FIG. 1b shows an example of a graphic pattern (14) with a lead line (14a). For a 7 segment electrode display, each of the seven segment electrodes has a lead line. The lead line (14a) may be formed on the same side of the pattern (14) on the substrate layer or on the opposite side when a via (through hole) is present.
In order to hide them for esthetic reasons, the lead lines are often routed from the patterns to the driver and/or control elements through vias (through holes). However, construction of vias involves costly and complex manufacturing steps. In addition, routing conductive lead lines along the plane of a display between conductive materials and associated components in a single layer often has the risk that undesired electric fields may be established, or desired electric fields may be interfered, by virtue of potentials applied to such lead lines (i.e., the lead lines may act as electrodes, potentially affecting the migration of the charged pigment particles in one or more electrophoretic cells positioned near the lead lines).